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HTTP Alternative ServicesAkamaimnot@mnot.nethttp://www.mnot.net/Mozillamcmanus@ducksong.comhttps://mozillians.org/u/pmcmanus/greenbytes GmbHjulian.reschke@greenbytes.dehttp://greenbytes.de/tech/webdav/Applications
HTTPbisHTTP
This document specifies "alternative services" for HTTP, which allow an
origin's resources to be authoritatively available at a separate network
location, possibly accessed with a different protocol configuration.
Discussion of this draft takes place on the HTTPBIS working group mailing list
(ietf-http-wg@w3.org), which is archived at .
Working Group information can be found at ; that specific to HTTP/2 are at .
The changes in this draft are summarized in .
HTTP conflates the identification of
resources with their location. In other words, "http://" (and "https://") URLs are
used to both name and find things to interact with.
In some cases, it is desirable to separate these aspects; to be able to keep
the same identifier for a resource, but interact with it using a different
location on the network.
For example:
An origin server might wish to redirect a client to an alternative when it
needs to go down for maintenance, or it has found an alternative in a
location that is more local to the client.
An origin server might wish to offer access to its resources using a new
protocol (such as HTTP/2, see ) or one using improved
security (such as TLS, see ).
An origin server might wish to segment its clients into groups of
capabilities, such as those supporting Server Name Indication (SNI,
see ) and those not
supporting it, for operational purposes.
This specification defines a new concept in HTTP, "Alternative Services", that
allows a resource to nominate additional means of interacting with it on the
network. It defines a general framework for this in , along with
a specific mechanism for discovering them using HTTP header fields in .
It also introduces a new status code in , so that origin servers (or
their nominated alternatives) can indicate that they are not authoritative for
a given origin, in cases where the wrong location is used.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in .
This document uses the Augmented BNF defined in
along with the "OWS", "DIGIT", "DQUOTE", "parameter", "uri-host", "port" and
"delta-second" rules from , and uses the "#rule"
extension defined in of that document.
This specification defines a new concept in HTTP, the "alternative service".
When an origin (see ) has resources that are accessible through a
different protocol / host / port combination, it is said to have an alternative
service.
An alternative service can be used to interact with the resources on an origin
server at a separate location on the network, possibly using a different
protocol configuration. Alternative services are considered authoritative for
an origin's resources, in the sense of .
For example, an origin:
("http", "www.example.com", "80")
might declare that its resources are also accessible at the alternative service:
("h2", "new.example.com", "81")
By their nature, alternative services are explicitly at the granularity of an
origin; i.e., they cannot be selectively applied to resources within an origin.
Alternative services do not replace or change the origin for any given resource;
in general, they are not visible to the software "above" the access mechanism.
The alternative service is essentially alternative routing information that can
also be used to reach the origin in the same way that DNS CNAME or SRV records
define routing information at the name resolution level. Each origin maps to a
set of these routes - the default route is derived from origin itself and the
other routes are introduced based on alternative-protocol information.
Furthermore, it is important to note that the first member of an alternative
service tuple is different from the "scheme" component of an origin; it is more
specific, identifying not only the major version of the protocol being used,
but potentially communication options for that protocol.
This means that clients using an alternative service will change the host, port
and protocol that they are using to fetch resources, but these changes &MUST-NOT;
be propagated to the application that is using HTTP; from that standpoint, the
URI being accessed and all information derived from it (scheme, host, port) are
the same as before.
Importantly, this includes its security context; in particular, when TLS
is in use, the alternative server will need to present a certificate
for the origin's host name, not that of the alternative. Likewise, the Host
header field is still derived from the origin, not the alternative service (just as it
would if a CNAME were being used).
The changes &MAY;, however, be made visible in debugging tools, consoles, etc.
Formally, an alternative service is identified by the combination of:
An ALPN protocol, as per A host, as per A port, as per
Additionally, each alternative service &MUST; have:
A freshness lifetime, expressed in seconds; see
There are many ways that a client could discover the alternative service(s)
associated with an origin.
Clients &MUST-NOT; use alternative services with a host other than the origin's
without strong server authentication; this mitigates the attack described in
. One way to achieve this is for the alternative to use TLS
with a certificate that is valid for that origin.
For example, if the origin's host is "www.example.com" and an alternative is
offered on "other.example.com" with the "h2" protocol, and the certificate
offered is valid for "www.example.com", the client can use the alternative.
However, if "other.example.com" is offered with the "h2c" protocol, the client
cannot use it, because there is no mechanism in that protocol to establish
strong server authentication.
Furthermore, this means that the HTTP Host header field and the SNI information
provided in TLS by the client will be that of the origin, not the alternative.
Mechanisms for discovering alternative services can associate a freshness
lifetime with them; for example, the Alt-Svc header field uses the "ma"
parameter.
Clients &MAY; choose to use an alternative service instead of the origin at any
time when it is considered fresh; see for specific
recommendations.
Clients with existing connections to alternative services are not required to
fall back to the origin when its freshness lifetime ends; i.e., the caching
mechanism is intended for limiting how long an alternative service can be used
for establishing new requests, not limiting the use of existing ones.
To mitigate risks associated with caching compromised values (see
for details), user agents &SHOULD; examine cached alternative
services when they detect a change in network configuration, and remove any
that could be compromised (for example, those whose association with the trust
root is questionable). UAs that do not have a means of detecting network
changes &SHOULD; place an upper bound on their lifetime.
A client must only use a TLS-based alternative service if the client also
supports TLS Server Name Indication (SNI) ().
This supports the conservation of IP addresses on the alternative service host.
By their nature, alternative services are optional; clients are not required to
use them. However, it is advantageous for clients to behave in a predictable
way when they are used by servers (e.g., for load balancing).
Therefore, if a client becomes aware of an alternative service, the client
&SHOULD; use that alternative service for all requests to the associated origin
as soon as it is available, provided that the security properties of the
alternative service protocol are desirable, as compared to the existing
connection.
When a client uses an alternate service, it &MUST; emit the Service header field
() on every request using that alternate service.
The client is not required to block requests; the origin's connection can be
used until the alternative connection is established. However, if the security
properties of the existing connection are weak (e.g. cleartext HTTP/1.1) then
it might make sense to block until the new connection is fully available in
order to avoid information leakage.
Furthermore, if the connection to the alternative service fails or is
unresponsive, the client &MAY; fall back to using the origin. Note, however, that
this could be the basis of a downgrade attack, thus losing any enhanced
security properties of the alternative service.
An HTTP(S) origin server can advertise the availability of alternative services
(see ) to clients by adding an Alt-Svc header field
to responses.
Alt-Svc = 1#( alternative *( OWS ";" OWS parameter ) )
alternative = DQUOTE protocol-id DQUOTE "=" port
protocol-id = <ALPN protocol identifier>
For example:
Alt-Svc: "http2"=8000
This indicates that the "http2" protocol on the same host using the
indicated port (in this case, 8000).
Alt-Svc &MAY; occur in any HTTP response message, regardless of the status code.
Alt-Svc does not allow advertisement of alternative services on other hosts, to
protect against various header-based attacks.
It can, however, have multiple values:
Alt-Svc: "h2c"=8000, "h2"=443
The value(s) advertised by Alt-Svc can be used by clients to open a new
connection to one or more alternative services immediately, or simultaneously
with subsequent requests on the same connection.
Intermediaries &MUST-NOT; change or append Alt-Svc values.
Finally, note that while it may be technically possible to put content other
than printable ASCII in a HTTP header field, some implementations only support ASCII
(or a superset of it) in header field values. Therefore, this field &SHOULD-NOT;
be used to convey protocol identifiers that are not printable ASCII, or those
that contain quote characters.
The header field syntax is both misleading (use of double quotes
although not a quoted string) and incomplete (does not support all values). Alternate
proposal in .
When an alternative service is advertised using Alt-Svc, it is considered fresh
for 24 hours from generation of the message. This can be modified with the 'ma'
(max-age) parameter;
Alt-Svc: "h2"=443;ma=3600
which indicates the number of seconds since the response was generated the
alternative service is considered fresh for.
ma = delta-seconds
See for details of determining
response age.
For example, a response:
HTTP/1.1 200 OK
Content-Type: text/html
Cache-Control: 600
Age: 30
Alt-Svc: "h2c"=8000; ma=60
indicates that an alternative service is available and usable for the next 60
seconds. However, the response has already been cached for 30 seconds (as per
the Age header field value), so therefore the alternative service is only fresh
for the 30 seconds from when this response was received, minus estimated
transit time.
When an Alt-Svc response header field is received from an origin, its value
invalidates and replaces all cached alternative services for that origin.
See for general requirements on caching alternative services.
Note that the freshness lifetime for HTTP caching (here, 600 seconds) does not
affect caching of Alt-Svc values.
The Service HTTP header field is used in requests to indicate the identity of the
alternate service in use, just as the Host header field identifies the host and port
of the origin.
Service = uri-host [ ":" port ]
Service is intended to allow alternate services to detect loops, differentiate
traffic for purposes of load balancing, and generally to ensure that it is
possible to identify the intended destination of traffic, since introducing
this information after a protocol is in use has proven to be problematic.
When using an Alternate Service, clients &MUST; include a Service header in all
requests.
For example:
GET /thing
Host: origin.example.com
Service: alternate.example.net
User-Agent: Example/1.0
The 4NN (Not Authoritative) status code indicates that the current origin
server (usually, but not always an alternative service; see ) is
not authoritative for the requested resource, in the sense of
.
Clients receiving 4NN (Not Authoritative) from an alternative service &MUST;
remove the corresponding entry from its alternative service cache (see
) for that origin. Regardless of the idempotency of the request
method, they &MAY; retry the request, either at another alternative server, or at
the origin.
4NN (Not Authoritative) &MAY; carry an Alt-Svc header field.
This status code &MUST-NOT; be generated by proxies.
A 4NN response is cacheable by default; i.e., unless otherwise indicated by the
method definition or explicit cache controls (see ).
We should decide on the status code before Last Call.
This document registers Alt-Svc in the Permanent Message Header Registry
.
Header Field Name: Alt-SvcApplication Protocol: httpStatus: standardAuthor/Change Controller: IETFSpecification Document: [this document]Related Information:
This document registers Alt-Svc in the Permanent Message Header Registry
.
Header Field Name: ServiceApplication Protocol: httpStatus: standardAuthor/Change Controller: IETFSpecification Document: [this document]Related Information:
This document registers the 4NN (Not Authoritative) HTTP Status code
().
Status Code: 4NNShort Description: Not AuthoritativeSpecification: [this document],
Identified security considerations should be enumerated in the appropriate
documents depending on which proposals are accepted. Those listed below are
generic to all uses of alternative services; more specific ones might be
necessary.
Using an alternative service implies accessing an origin's resources on an
alternative port, at a minimum. An attacker that can inject alternative services
and listen at the advertised port is therefore able to hijack an origin.
For example, an attacker that can add HTTP response header fields can redirect
traffic to a different port on the same host using the Alt-Svc header field; if
that port is under the attacker's control, they can thus masquerade as the HTTP
server.
This risk can be mitigated by restricting the ability to advertise alternative
services, and restricting who can open a port for listening on that host.
When the host is changed due to the use of an alternative service, it presents
an opportunity for attackers to hijack communication to an origin.
For example, if an attacker can convince a user agent to send all traffic for
"innocent.example.org" to "evil.example.com" by successfully associating it as
an alternative service, they can masquerade as that origin. This can be done
locally (see mitigations above) or remotely (e.g., by an intermediary as a
man-in-the-middle attack).
This is the reason for the requirement in that any alternative
service with a host different to the origin's be strongly authenticated with
the origin's identity; i.e., presenting a certificate for the origin proves
that the alternative service is authorized to serve traffic for the origin.
However, this authorization is only as strong as the method used to
authenticate the alternative service. In particular, there are well-known
exploits to make an attacker's certificate appear as legitimate.
Alternative services could be used to persist such an attack; for example, an
intermediary could man-in-the-middle TLS-protected communication to a target,
and then direct all traffic to an alternative service with a large freshness
lifetime, so that the user agent still directs traffic to the attacker even when
not using the intermediary.
As a result, there is a requirement in to examine cached alternative
services when a network change is detected.
When the ALPN protocol is changed due to the use of an alternative service, the
security properties of the new connection to the origin can be different from
that of the "normal" connection to the origin, because the protocol identifier
itself implies this.
For example, if a "https://" URI had a protocol advertised that does not use
some form of end-to-end encryption (most likely, TLS), it violates the
expectations for security that the URI scheme implies.
Therefore, clients cannot blindly use alternative services, but instead evaluate
the option(s) presented to assure that security requirements and expectations
(of specifications, implementations and end users) are met.
Thanks to Eliot Lear, Stephen Farrell, Guy Podjarny, Stephen Ludin, Erik
Nygren, Paul Hoffman, Adam Langley, Will Chan and Richard Barnes for their
feedback and suggestions.
The Alt-Svc header field was influenced by the design of the
Alternative-Protocol header field in SPDY.
Key words for use in RFCs to Indicate Requirement LevelsHarvard UniversityUniform Resource Identifier (URI): Generic SyntaxWorld Wide Web ConsortiumDay SoftwareAdobe Systems IncorporatedAugmented BNF for Syntax Specifications: ABNFTransport Layer Security (TLS) Extensions: Extension DefinitionsThe Web Origin ConceptTransport Layer Security (TLS) Application Layer Protocol Negotiation ExtensionHypertext Transfer Protocol (HTTP/1.1): Message Syntax and RoutingAdobe Systems Incorporatedfielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol (HTTP/1.1): CachingAdobe Systems Incorporatedfielding@gbiv.comAkamaimnot@mnot.netgreenbytes GmbHjulian.reschke@greenbytes.deRegistration Procedures for Message Header FieldsThe Transport Layer Security (TLS) Protocol Version 1.2Hypertext Transfer Protocol (HTTP/1.1): Semantics and ContentAdobe Systems Incorporatedfielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol version 2
This is the first version after adoption of draft-nottingham-httpbis-alt-svc-05
as Working Group work item. It only contains editorial changes.